1. Field of the Invention
This invention relates to a process for the preparation of stable aqueous dispersions of polyurethane-polyureas, to the dispersions obtained and to their use for the production of coatings.
2. Description of the Prior Art
Polyurethane-polyureas have numerous commercial applications, for example as adhesives or coating compounds for various substrates such as textiles, plastics, wood, glass fibers, leather and metals. Chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability are some of the possible properties of these materials.
It is known in the art that polyurethanepolyureas may be applied as coating compounds or adhesives from solutions in organic solvents. This is economically disadvantageous due to the high cost of the solvents, but even more important is the atmospheric pollution caused by the solvents.
Numerous experiments have therefore been undertaken to produce polyurethane-polyurea coatings from dispersions of these polymers in water. This procedure is economical since water is used as solvent and causes no atmospheric pollution. Polyurethanepolyureas are, however, incompatible with water, i.e. they do not form stable dispersions in water unless they are produced by special manufacturing processes or with the aid of special additives.
In earlier methods for the preparation of stable dispersions of polyurethane-polyureas, external emulsifiers were used for dispersing and stabilizing the polymers in water, for example as described in U.S. Pat. No. 2,968,575. Although dispersions could be prepared by this method, the coatings obtained had serious disadvantages, for example a high sensitivity to water due to the presence of these emulsifiers.
In subsequent developments, emulsifiers which could be chemically incorporated in the polyurethanepolyurea chain were used. Such ionic emulsifiers are described, for example, in U.S. Pat. No. 3,479,310. The use of chemically incorporated nonionic emulsifiers is described, for example, in U.S. Pat. Nos. 3,905,929, 3,920,598 and 4,190,566. Further improvements in the properties of coatings obtained from polyurethane-polyurea dispersions were achieved by the combination of ionic and nonionic starting components, as described, for example, in U.S. Pat. Nos. 4,092,286, 4,237,264 and 4,238,378.
These developments led to dispersions with improved stability and lower proportions of hydrophilic starting components. Since these dispersions contained a lower proportion of hydrophilic starting components, they could be used for the formation of coatings with improved water resistance and less loss of mechanical strength in the swelled state. The preparation of polyurethane-polyurea dispersions obtained by dispersing prepolymers containing isocyanate end groups in water and then chain lengthening with diamines is described in U.S. Pat. No. 4,066,591. The prepolymers containing ionic groups mentioned in U.S. Pat. No. 4,066,591 are obtained by a simple method of reacting an excess of isocyanate groups with starting components containing isocyanate-reactive groups and ionic or potentially ionic groups until the theoretical amount of free isocyanate groups are obtained. No particular sequence of reactions of the starting components is described.
U.S. Pat. No. 4,408,008 also describes the preparation of prepolymers containing isocyanate groups followed by dispersion in water. In this case the prepolymers are prepared from organic polyisocyanates, compounds containing at least two isocyanate-reactive groups, organic compounds containing hydrophilic ethylene oxide units and optionally organic compounds containing ionic or potential ionic groups.
Dispersions of tertiary amine salts of polyurethane-ureas prepared from polyamines such as triamines and urethane prepolymers containing isocyanate end groups and carboxyl groups are described in U.S. Pat. No. 4,203,883. In accordance with this invention, the isocyanate-terminated prepolymers may be prepared by the reaction of isocyanates and polyols in a simultaneous or a stepwise process. The said disclosure does not mention that the stepwise method of reaction has any particular influence on the properties of the resulting dispersions.
Cross-linked polyurethane dispersions obtained from isocyanate-terminated prepolymers containing carboxylate groups are described in U.S. Pat. No. 4,554,308. Examples are given in which the carboxyl groups are incorporated in the prepolymer in the second step of a stepwise reaction sequence. Here again, there is no mention of the reaction method having any particular influence on the properties.
One disadvantage of the known polyurethanepolyurea dispersions is that a relatively large quantity of ionic groups must be incorporated if good dispersibility is to be achieved.
It is an object of the present invention to provide an improved process for the preparation of polyurethane-polyurea dispersions in which the quantity of ionic groups required for dispersion is reduced without sacrificing the other desirable properties of coatings or films prepared from these dispersions.